This invention arose out of problems associated with tungsten plug formation in contacts formed through silicon dioxide insulating layers. The problem is best understood with reference to the accompanying FIGS. 1 and 2. There illustrated is a semiconductor wafer fragment 10 comprised of a bulk substrate 12 and an overlying silicon dioxide layer 14, such as borophosphosilicate glass (BPSG). Bulk substrate 12 includes a dopant diffusion/active region 16 to which electrical connection is to be made. A contact opening 18 is provided through BPSG layer 14 to active area 16.
A thin layer 20 of titanium is deposited atop the wafer to within contact opening 18. Titanium layer 20 is provided to function as a silicide formation layer at the base of contact 18 for reducing resistance. An undesired oxide layer (not shown) also typically forms atop diffusion region, 16. The deposited elemental titanium also functions to break-up this undesired oxide and thereafter form a titanium silicide with the silicon of substrate 12 to reduce contact resistance between active area 16 and subsequently deposited plug filling tungsten. Additionally, titanium layer 20 functions as an adhesion/nucleation layer for the subsequently deposited tungsten. Tungsten does not readily deposit over silicon dioxide and exposed silicon substrate, and the intervening titanium layer 20 facilitates deposition and adhesion of tungsten thereto.
Titanium layer 20 is typically deposited by sputter deposition, and undesirably results in formation of contact projecting cusps 22. This results in a back or re-entrant angle 24 being formed relative to contact opening 18. A layer 26 of tungsten is subsequently deposited with the intent being to completely fill the remaining volume of contact opening 18. Unfortunately, an undesired keyhole 28 typically forms, leaving a void within contact 18.
Referring to FIG. 2, layers 26 and 20 are subsequently etched back by dry etch of chemical-mechanical polishing to form a contact-filling plug 30. Undesirably, this typically opens-up the upper end of keyhole 28. In processing, this undesirably creates a thin void which is difficult to clean and rinse during processing. Also in the final construction, the outer surface area of plug 30 is reduced due to the void created by keyhole 28. This counters the desired goal of maximizing electrical contact with plug 30 with a subsequent layer for ultimately making electrical connection with active area 16. Further, the etch back typically conducted to produce plug 30 undesirably over-etches titanium layer 20, forming edge "fangs" 32.
One prior art solution for overcoming this problem is described with reference to FIG. 3. Like numerals are utilized where appropriate, such that only differences are described and numbered with a suffix "a". Here, BPSG layer 14 is angle-etched from its outer surface downwardly to active area 16. Subsequently deposited titanium layer 20 therefore does not form cusps, which will result in elimination of a keyhole by the subsequently deposited tungsten layer. This method is, however, highly undesirable as contact opening 18a has the added drawback of being narrower at its base than the illustrated FIGS. 1 and 2 embodiment, and also significantly wider at its outermost portion. This undesirably consumes precious wafer area and thereby counters the continuing semiconductor processing goals of making device components smaller and smaller.
It would be desirable to overcome these and other problems associated with formation of electrically conductive contact plugs. Although the invention principally arose out of concern specific to tungsten plug formation, the artisan will appreciate that the invention has applicability with other materials and constructions. The invention is intended to only be limited by the accompanying claims appropriately interpreted in accordance with the Doctrine of Equivalents.